The present invention generally relates to electrical power in a vehicle. More particularly, the present invention relates to vehicular electric machines (starters, generators and starter-generators).
In some vehicles, it has been found desirable to insert an electric machine directly between an output shaft of an engine and a transmission of the vehicle. This may be a particularly desirable arrangement when the vehicle has on-board electrical loads which require large amounts of power and the electric machine may have a large electrical output capacity. In such an arrangement, the electric machine may be positioned so that it consumes minimal space in an engine/transmission envelope. In other words, the electric machine may be positioned coaxially with a drive shaft of the engine and may not need to be placed alongside the engine. Typically such coaxially positioned electric machines are constructed as permanent magnet (PM) electric machines. PM electric machines may produce power with a voltage that varies as a function of their rotational speed and the electrical load. When such machines are driven by a variable speed engine, their output must be conditioned prior to being delivered to electrical loads.
In some vehicle electrical systems, it is desirable to employ wound field starter-generators. Such wound field machines may produce output power that may be controlled to a fixed voltage even though rotational speed of, and/or the electrical load on, the wound field machine may vary. Wound field machines may have advantages in vehicular applications because they may output power that may not require use of power conditioning equipment. Thus overall weight and cost of a wound-field based electrical system may be lower than that of a PM-based electrical system because there may be no need for on-board power conditioning equipment.
In spite of some desirable features of wound-field systems, PM systems have heretofore been employed in applications that require a coaxial arrangement of a starter-generator with its respective engine output shaft. This is because a typical PM machine may be constructed with a smaller axial length than a typical wound field machine. To achieve brushless operation a typical wound field machine may consist of an exciter generator and a main generator. Thus two generators may consume side-by-side space in many wound-field machine designs. Some newly developed wound field machines have their exciter generators and their main generators concentrically arranged in a so-called “low profile” configuration. Such a machine is described in U.S. Pat. No. 7,230,363, which is incorporated by reference herein.
While the concentrically arranged wound-field machine of U.S. Pat. No. 7,230,363 may have a desirably small envelope, it nevertheless is not configured for positioning between an engine and a transmission coaxially with an output shaft of an engine. This is because the machine of U.S. Pat. No. 7,230,363 does not have a configuration for transferring mechanical power to the vehicle transmission.
As can be seen, there is a need for an electrical system that employs a low-profile, wound field electric machine which may be positioned between an engine output shaft and a transmission of the vehicle.